The present invention concerns improvements in a manufacturing process, and more especially concerns apparatus and methods suitable for the manufacture of catalysts supported on monolithic supports and like products.
Large numbers of catalysts supported on high surface area monolithic supports are manufactured each year. One of the principal types of such catalyst is the catalytic convertor for the internal combustion engine, and the present invention has particular utility in relation to these. For conciseness, the following description will particularly describe the use of the invention for car catalysts, but it should be understood that the invention may find application in all similar types of catalyst, whether for vehicles (xe2x80x9ccar catalystsxe2x80x9d is intended to include trucks, utility vehicles, buses, motorcycles etc), ships, stationary power sources or gas clean-up from other industrial processes. The present invention is also useful in the manufacturing of like products where a coating is applied to a carrier, and we mention particularly absorber/desorber coatings, many of which are used in exhaust emission control. Such products are not technically xe2x80x9ccatalystsxe2x80x9d. We would mention particularly NOX absorbers, hydrocarbon absorbers, absorbers for sulphur compounds, and water vapour absorbers. As well as absorbers for gas treatment, absorbers may find use in heat pumps, eg air conditioning coolers.
The car catalyst is generally supported on an extruded ceramic, eg cordierite, or manufactured metal through-flow honeycomb substrate. The substrates are generally cylindrical, but may be oval or xe2x80x9cracetrackxe2x80x9d or a skewed oval, and have an effective continuous outer skin (if the support or substrate does not have a continuous skin, the present invention may be used if the support or substrate is located within a sleeve.) Such substrates arc very well known and are commercially available, and may have from 50 to 1200 cells/sq in. In order to increase the surface area for catalysis, it is commonplace to coat the substrate with a washcoat, comprising a slurry of high surface area particles such as alumina, optionally containing other components such as soluble and/or supported catalytically active platinum group metals (xe2x80x9cPOMsxe2x80x9d), promoters such as ceria, zirconia, barium etc and other components or absorptive materials such as silica, zeolitc etc. In some cases, after the washcoat is applied and dried and/or fired to give an adherent coating, one or more catalyst layers is applied. This may be by impregnation of solutions of one or more PGMs, generally selected from one or more of platinum, palladium and rhodium, and/or soluble promoters and/or by applying more layers of the same or different types of catalytically active or absorptive washcoats. The coating processes in use are usually considered as confidential know-how, but are generally variations on immersing the substrate in the slurry or solution, or passing the substrates through a curtain or waterfall of the slurry or solution and using compressed air to blow the fluid into and through the substrate, ensuring coating of the cells and also ensuring that there are no or practically no blocked cells.
We have realised that such processes are not very flexible and have a number of other disadvantages. The slurry or solution is recycled, but the absorption characteristics of the substrate or washcoated substrate are such that certain components, eg finer particles or certain dissolved platinum group metal salts, deposit preferentially, thus changing the composition of the recycled fluid and accordingly altering some of the characteristics of the subsequently coated catalysts. We have accordingly realised that continuous processes may not provide the best answer for producing catalysts with identical performance and other characteristics. The present invention provides a batch or semi-batch process that offers very significant improvements in producing car catalysts, with advantages in consistency and control of raw materials, as well as offering catalyst design options not previously available for large scale manufacturing. It is believed that the present invention may have particular applicability in the coating of very high cell density substrates which are difficult to coat using conventional technology.
We are aware of some prior proposals which have some of the objectives of the present invention. For example, in U.S. Pat. No. 5,543,181, catalyst solution is sprayed into the cells of a honeycomb support through probes which are inserted into the cells. Such a system is mechanically complex and not capable of coping with very high cell density substrates. U.S. Pat. No. 4,931,419 discloses a method of soaking one end of a monolithic support in a specified volume of liquid, equal to the pore volume of the support and turning the monolithic support over to cause the liquid to drain through the support. Such a method still permits losses of valuable precious metal catalysts. U.S. Pat. No. 5,422,138 teaches a coating apparatus involving dipping the base of a monolithic support into a coating liquid then blowing excess liquid from the cells. WO 97/48500 describes a further catalyst coating system where a substrate is partially immersed in a coating liquid, a vacuum is applied to draw the liquid up into the cells of the substrate to a distance less than the length of the cells Such a system is not very flexible and still recycles coating liquid.
A further proposal is disclosed in GB 2,012,616, which discloses apparatus and methods for coating catalyst substrates using a measured charge of slurry which is drawn into and through the substrate. It is clear that the charge of slurry is such that it is in excess of that actually required to coat the substrate. It is not believed that this proposal was ever developed to the stage of commercial utility. We believe that the fact that excess slurry is used, which is recycled for re-use, results in the alteration of composition in time as various components of the slurry deposit preferentially onto the walls of the substrate.
The present invention provides a monolithic support coating apparatus, comprising means for dosing a predetermined quantity of a liquid component such quantity being such that it is substantially wholly retained within the intended support, liquid component containment means locatable on the top of a support to receive said quantity of liquid component and pressure means capable of withdrawing the liquid component from the containment means into at least a portion of the support.
The invention further provides a method of coating a monolithic support, comprising the steps (a) locating a containment means on top of a support, (b) dosing a predetermined quantity of a liquid component into said containment means, either in the order (a) then (b) or (b) then (a), and (c) by applying pressure or vacuum, drawing said liquid component into at least a portion of the support, and retaining substantially all of said quantity within the support. The method may, but need not, provide plug flow into the support.
Desirably, no more than 1 wt % of the liquid, more preferably no more than 0.5%, especially none of the liquid, is drawn through the substrate and expelled.
The liquid component may be any liquid used to coat a monolithic support, whether or not it contains catalytic component In general, this will be a slurry of particles, eg absorber or a washcoat slurry, or a solution of catalytically active components although for certain advanced design catalysts, a washcoat slurry may contain catalytically active particles or solutions. The actual composition of the liquid component is not important for the present invention.
Part of the present invention is a liquid component containment means which is located on top of a support to be coated. Whilst the design and function of this will be better appreciated from the specific description hereinafter, it should be understood to comprise at least a wall generally corresponding to the cross-sectional shape of the support. It may also comprise a permeable base closing the walls to form a container. The wall may be manufactured from plastics, including polypropylene or preferably PTFE, metal, especially stainless steel or PTFE-coated stainless steel, or ceramic, and the base may be a perforated or porous metal plate, sintered metal sponge, a woven or non-woven fabric or a synthetic open cell foam. In certain embodiments, the base or part thereof may be moveable in the manner of a guillotine, iris or shutter, or a separate permeable base, for example applied to the face of the support, may be used. Combinations of different materials may be used, for example, the foam is useful to distribute the liquid evenly across the face of the support, and hence ensure even coating. For certain coating components, the rheology may be such that the component does not begin to enter the cells of the support until pressure or vacuum is applied, and hence no permeable base is required to contain the liquid component The containment means need not, and in certain preferred embodiments is designed not to, contain and hence locate the liquid component over the entire upper surface area of the support The containment means may be shaped to blank off areas of the support, so that coating is not carried out in those areas. For example, in a car catalyst or absorber which is mounted in a housing in the exhaust, the support is sealed around its edges to prevent gas flow by-passing the support. The effect is to seal off a circumferential quantity of cells, so that these cells are not used for catalysis or absorption/desorption and therefore their content of precious metal catalyst or absorber may be considered to be wasted. The present invention offers the possibility of not coating such circumferential cells. It is also well understood that for a variety of reasons, gas flow is concentrated in the central portion of a monolith, and the present invention permits such a central portion to be coated with a different composition and/or different concentration of components. Accordingly, the containment means may be divided internally to separate different liquids for coating, or parts may be blanked off to prevent coating. It is within the present invention to modify the base of the containment means to achieve a desired coating profile or to yield a desired product. For example, the base may have differing permeability to give easier flow, and hence greater coating deposits, in certain regions, or the base may itself act as a profiled containment means, eg by machining a honeycomb monolith, so that certain regions contain greater quantities of liquid component which are transferred directly to corresponding parts of the support located below the base.
It will be understood that the containment means is desirably provided with sealing means effective to prevent loss of coating liquid when pressure/vacuum is applied. Such scaling may be achieved by using a deformable pemeable base, or by a separate seal. One preferred type of seal is an inflatable seal, which may be inflated and deflated under machine control. An inflatable seal permits the apparatus to cater for variations in substrate diameter or circumference. If all the cells of a honeycomb support, or all the area of any other type of support, require to be coated, it is not possible to seal on the top face of the support and accordingly a suitable seal against the outer side of the support enables the coating liquid to access the whole of the support.
It is possible to conceive of the development of accurate dosing means which together with a gel-like liquid component, deposits the coating component onto a support without requiring containment means. This modification of the present invention may be considered as using the structure of the liquid component itself as the containment means and hence is within the scope of the invention.
The liquid dosing means may be engineered using generally known principles. For example, there are commercially available machines capable of highly accurate dosing by volume or weight of liquids, for example used for filling in the food or paint industry. These machines are capable of handling liquids of many different viscosities and rheologies. We believe that the accuracy of dosing using current technology is xc2x10.1 wt %. This permits a very significant improvement in coating accuracy according to the preset invention, it should also be mentioned that the accuracy of the coating process is also dependent upon the accuracy of the liquid (solution or slurry) analysis. However, the invention offers greatly increased opportunities for control of all aspects of coating.
It is desirable to combine the dosing means with automatic handling equipment, which may include a rotatable indexed table capable of moving supports past a number of work stations at which the various stages of the method may take place. Robotic handling may be provided to load and unload supports, and it is envisaged that the method may be used to part-coat the support in the axial direction, then turn the part-coated support and coat the remainder from the other face. It will be appreciated that the basic apparatus is extremely flexible, and can be designed to have easily changeable dosing means and containment means, and a number of different coating functions may be carried out in the same apparatus. Of course, the invention includes essentially manual operation, which may be appropriate for low volume, low labour cost applications.
In a further embodiment of the present invention, the dosing head and containment means are combined into a unitary structure which may be located on top of the support, and either moved into fluid connection with a source of the coating liquid, or is permanently connected thereto, possibly using flexible lines to permit movement of the dosing head and/or the support.
The apparatus and method of the invention may provide plug flow of the dispensed quantity of liquid into the support. An important characteristic of the invention is that substantially all of the liquid is retained within the support ensuring even distribution in the desired pattern, and ensuring that there is no significant excess liquid requiring recycling.
Any small quantities of liquid which are not retained within the support are captured in the preferred embodiments in the vacuum hood as illustrated hereinafter, thus improving the health and safety environment.
Although the invention may operate using air pressure or a combination of air pressure and vacuum to initiate transfer of the liquid from the containment means into the support, we presently prefer to use the application of a vacuum to the bottom of the support, causing normal air pressure to force the liquid into the cells of the support. If necessary, a pulse of air flow may be initiated to clear the cells of the support and prevent blockages. In more advanced embodiments, the vacuum itself may be non-uniformly applied, either temporally or spatially.
The quantity of liquid component is predetermined according to the desired coating design and the behaviour of the liquid component to coat the support without excess, by conventional experimental methods. The liquid is desirably stored in a large volume container which may be stirred and will be replenished from time-to-time. Since the apparatus provides accurately measured volumes and no recycling, the quality or composition of the liquid component remains constant, and losses are reduced.
A particular advantage of the invention in the manufacture of catalysts is that usage of precious metal is very well controlled, and it is not so necessary to over-specify metal loadings in order to cope with process variations.
After coatings the support may be dried, and fired if required, to fix or transform the coating.